Cutter element with non-rectilinear crest

ABSTRACT

A cutter element for a drill bit. The cutter element has a non-rectilinear crest. The non-rectilinear or curvilinear crest provides an advantageous distribution of the cutting forces across the body of the cutter elements and thus improves bit life. The curvilinear crest also allows the cutter element to more efficiently lift the portion of the formation that is being cut, thereby improving cutting action in certain formations. The cutter elements can have either positive or non-positive draft and can be tungsten carbide inserts.

CROSS-REFERENCE TO RELATED APPLICATION

The present application claims the benefit of 35 U.S.C. 111(b)provisional application Serial No. 60/057,915 filed Sep. 4, 1997 andentitled Cutter Element with Expanded Crest Geometry.

FIELD OF THE INVENTION

The invention relates generally to earth-boring bits used to drill aborehole for the ultimate recovery of oil, gas or minerals. Moreparticularly, the invention relates to rolling cone rock bits and to animproved cutting structure for such bits. Still more particularly, theinvention relates to a cutter element having a crest that is not astraight line when viewed along the longitudinal axis of the cutterelement.

BACKGROUND OF THE INVENTION

An earth-boring drill bit is typically mounted on the lower end of adrill string and is rotated by rotating the drill string at the surfaceor by actuation of downhole motors or turbines, or by both methods. Withweight applied to the drill string, the rotating drill bit engages theearthen formation and proceeds to form a borehole along a predeterminedpath toward a target zone. The borehole formed in the drilling processwill have a diameter generally equal to the diameter or "gage" of thedrill bit.

A typical earth-boring bit includes one or more rotatable cutters thatperform their cutting function due to the rolling movement of thecutters acting against the formation material. The cutters roll andslide upon the bottom of the borehole as the bit is rotated, the cuttersthereby engaging and disintegrating the formation material in its path.The rotatable cutters may be described as generally conical in shape andare therefore sometimes referred to as rolling cones. Such bitstypically include a bit body with a plurality of journal segment legs.The rolling cone cutters are mounted on bearing pin shafts that extenddownwardly and inwardly from the journal segment legs. The borehole isformed as the gouging and scraping or crushing and chipping action ofthe rotary cones remove chips of formation material which are carriedupward and out of the borehole by drilling fluid which is pumpeddownwardly through the drill pipe and out of the bit.

The earth-disintegrating action of the rolling cone cutters is enhancedby providing the cutters with a plurality of cutter elements. Cutterelements are generally two types: inserts formed of a very hardmaterial, such as cemented tungsten carbide, that are press fit intoundersized apertures or similarly secured in the cone surface; or teeththat are milled, cast or otherwise integrally formed from the materialof the rolling cone. Bits having tungsten carbide inserts are typicallyreferred to as "TCI" bits.

The cutting surfaces of inserts are, in some instances, coated with avery hard "superabrasive" coating such as polycrystalline diamond (PCD)or cubic boron nitride (PCBN). Superabrasive materials are significantlyharder than cemented tungsten carbide. As used herein, the term"superabrasive" means a material having a hardness of at least 2,700Knoop (kg/mm²). Conventional PCD grades have a hardness range of about5,000-8,000 Knoop, while PCBIN grades have a hardness range of about2,700-3,500 Knoop. By way of comparison, a typical cemented tungstencarbide grade used to form cutter elements has a hardness of about 1475Knoop. In each case, the cutter elements on the rotating cuttersfunctionally breakup the formation to create new borehole by acombination of gouging and scraping or chipping and crushing.

The cost of drilling a borehole is proportional to the length of time ittakes to drill to the desired depth and location. In oil and gasdrilling, the time required to drill the well, in turn, is greatlyaffected by the number of times the drill bit must be changed in orderto reach the targeted formation. This is the case because each time thebit is changed, the entire string of drill pipe, which may be mileslong, must be retrieved from the borehole, section by section. Once thedrill string has been retrieved and the new bit installed, the bit mustbe lowered to the bottom of the borehole on the drill string, whichagain must be constructed section by section. As is thus obvious, thisprocess, known as a "trip" of the drill string, requires considerabletime, effort and expense. Accordingly, it is always desirable to employdrill bits which will drill faster and longer and which are usable overa wider range of formation hardness.

The length of time that a drill bit may be employed before it must bechanged depends upon its rate of penetration ("ROP"), as well as itsdurability or ability to maintain an acceptable ROP. The form andpositioning of the cutter elements on the cone cutters greatly impactbit durability and ROP and thus are critical to the success of aparticular bit design.

Bit durability is, in part, measured by a bit's ability to "hold gage,"meaning its ability to maintain a full gage borehole diameter over theentire length of the borehole. To assist in maintaining the gage of aborehole, conventional rolling cone bits typically employ a heel row ofhard metal inserts on the heel surface of the rolling cone cutters. Theheel surface is a generally frustoconical surface and is configured andpositioned so as to generally align with and ream the sidewall of theborehole as the bit rotates. The inserts in the heel surface contact theborehole wall with a sliding motion and thus generally may be describedas scraping or reaming the borehole sidewall.

In addition to the heel row inserts, conventional bits typically includea primary "gage" row of cutter elements mounted adjacent to the heelsurface but oriented and sized so as to cut the corner as well as thebottom of the borehole. Conventional bits can also contain a secondarygage trimming row or a nestled gage row with lesser extension to assistin trimming the bore hole wall. Conventional bits also include a numberof additional rows of cutter elements that are located on the cones inrows disposed radially inward from the gage row. These cutter elementsare sized and configured for cutting the bottom of the borehole and aretypically described as primary "inner row" cutter elements. Together,the primary gage and primary inner row cutter elements of the bit formthe "primary rows." Primary row cutter elements are the cutter elementsthat project the most outwardly from the body of the rolling cone forcutting the bore hole bottom.

A review of post run bit performance data from 1991 through 1995indicated that most aggressive roller cone cutting structures from bothmilled tooth and tungsten carbide insert bits were sub-optimal ataddressing very soft rock formations (i.e. less than 2000 psi unconfinedrock compressive strength). Ultra-soft to soft formations typicallyconsist of clays, claystones, very soft shales, occasionally limy marls,and dispersed or unconsolidated sands, typically exhibit plasticbehavior. Very soft or weak clays/shales vary in their mechanicalresponse from more competent (harder) shales, under the same compressionloads, as applied in rotary rock bit drilling. Soft shales respondplastically, or simply deform under the applied load, as opposed to abrittle failure or rupture (crack) formed in more competent rocks tocreate the cutting or chip. In these very soft/plastic formationapplications, we cannot rely on conventional brittle rock failure modes,where cracks propagate from the loaded tooth penetration crater to theadjacent tooth craters, to create a chip or cutting. For this reason,the cutting structure arrangement must mechanically gouge away a largepercentage of the hole bottom in order to drill efficiently. In thesetypes of formations, maximum mechanical efficiency is accomplished bymaximizing the bottom hole coverage of the inserts contacting the holebottom per revolution so as to maximize the gouging and scraping action.

SUMMARY OF THE INVENTION

The present invention provides maximum scraping action and allowsgreater flexibility in the number of cutter elements used on a drillbit. According to the present invention, at least one cutter element ona bit is provided with a non-rectilinear crest. The term"non-rectilinear" is used to refer to configurations that are other thanstraight lines and includes curvilinear crests. In a preferredembodiment, at least a portion of the non-rectilinear crest is curved soas to improve the distribution of forces through the cutter element. Theconcepts of the present invention can be used in cutter elements thathave non-circular or non-cylindrical bases and can be used in tungstencarbide inserts and tungsten carbide inserts coated with superabrasives.

The present invention is also discloses the use of cutter elementshaving non-positive drafts. As used herein, the term "non-positivedraft" refers to the cutting portion of the cutting element extendingout to and beyond the envelope defined by the base portion.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects and advantages of the invention will become apparent uponreading the following detailed description and upon reference to theaccompanying drawings in which:

FIG. 1 is a perspective view of a conventional earth-boring bit;

FIG. 2 is a partial section view taken through one leg and one rollingcone cutter of the bit shown in FIG. 1;

FIGS. 3A-D are top, front, side and perspective views, respectively, ofa prior art chisel insert;

FIGS. 4A-C are top, front, and side views, respectively, of a prior artconical insert;

FIGS. 5A-C are top, front and side views, respectively, of a chiselinsert having one feature of the present invention;

FIGS. 6A-D are top, front, side and perspective views, respectively, ofa second type of novel chisel insert;

FIG. 3E shows the cutter elements of a prior art drill bit rotated intoa single plane;

FIG. 6E shows the cutter elements of FIG. 6A-D rotated into a singleplane;

FIGS. 7A-C are top, front and side views, respectively, of an offsetcrest chisel with a negative draft;

FIGS. 8A-C are top, front and side views, respectively, of an offsetcrest chisel with a negative draft and a reinforcement rib;

FIGS. 9A-C are top, front and side views, respectively, of an offsetconical insert with a negative draft;

FIGS. 10A-C are top, front and side views, respectively, of a biasednegative draft chisel insert;

FIGS. 11A-C are top, front and side views, respectively, of a partialbiased negative draft chisel insert;

FIGS. 12A-C are top, front and side views, respectively, of an arc crestchisel insert with zero draft;

FIGS. 13A-C are top, front and side views, respectively, of an arc crestchisel insert with negative draft;

FIGS. 14A-C are top, front and side views, respectively, of a spline orS-shaped crest chisel insert with zero draft;

FIGS. 15A-C are top, front and side views, respectively, of a spline orS-shaped crest chisel insert with negative draft;

FIGS. 16A-C are top, front and side views, respectively, of a partialnegative draft chisel insert;

FIGS. 17A-C are top, front and side views, respectively, of an offsetcrest chisel insert with negative draft on its leading flank;

FIGS. 18A-C are top, front and side views, respectively, of a slantcrest chisel insert with negative draft;

FIG. 19 is a simplified illustration of a prior art insert pressingtechnique;

FIG. 20 is a simplified illustration of an insert pressing technique inaccordance with the present invention;

FIG. 21 is a layout showing a first configuration of the cutter elementsof the present invention with respect to a projection of the roller coneaxis;

FIG. 22 is a layout showing an alternative configuration of the cutterelements of the present invention with respect to a projection of theroller cone axis;

FIG. 23 is a layout showing a second alternative configuration of thecutter elements of the present invention with respect to a projection ofthe roller cone axis;

FIG. 23A is a different view of the configuration of FIG. 23, lookingalong the axis of the cutter element and showing its orientation withrespect to a projection of the cone axis;

FIG. 24 is a layout showing a third alternative configuration of thecutter elements of the present invention with respect to a projection ofthe roller cone axis;

FIGS. 25A-D are top, front, side and perspective views, respectively, ofan arc crest chisel insert with positive draft;

FIGS. 26A-D are top, front, side and perspective views, respectively, ofa spline or S-shaped crest chisel insert with positive draft;

FIGS. 27A-D are top, front, side and perspective views, respectively, ofa spline or S-shaped crest chisel insert with positive draft in whichthe direction of the S is reversed as compared to the S shown in FIGS.26A-D;

FIGS. 28A-D are top, front, side and perspective views, respectively, ofa chisel insert having a J-shaped crest and a positive draft;

FIGS. 29A-D are top, front, side and perspective views, respectively, ofa chisel insert having a J-shaped crest and a positive draft in whichthe direction of the "J" is reversed as compared to the "J" shown inFIGS. 28A-D;

FIGS. 30A-D are top, front, side and perspective views, respectively, ofan insert having an arcuate crest that is not perpendicular to thelongitudinal axis of the insert; and

FIGS. 31A-D are top, front, side and perspective views, respectively, ofan insert having an arcuate crest and a concave leading face.

While the invention is susceptible to various modifications andalternative forms, specific embodiments thereof are shown by way ofexample in the drawings and are described in detail below. It should beunderstood, however, that the drawings and detailed description thereofare not intended to limit the invention to the particular formdisclosed, but on the contrary, the intention is to cover allmodifications, equivalents and alternatives falling within the spiritand scope of the present invention as defined by the appended claims.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring first to FIG. 1, an earth-boring bit 10 made in accordancewith the present invention includes a central axis 11 and a bit body 12having a threaded section 13 on its upper end for securing the bit tothe drill string (not shown). Bit 10 has a predetermined gage diameteras defined by three rolling cone cutters 14, 15, 16, which are rotatablymounted on bearing shafts that depend from the bit body 12. Bit body 12is composed of three sections or legs 19 (two shown in FIG. 1) that arewelded together to form bit body 12. Bit 10 further includes a pluralityof nozzles 18 that are provided for directing drilling fluid toward thebottom of the borehole and around cutters 14-16. Bit 10 further includeslubricant reservoirs 17 that supply lubricant to the bearings of each ofthe cutters.

Referring now to FIG. 2, in conjunction with FIG. 1, each cutter 14-16is rotatably mounted on a pin or journal 20, with an axis of rotation 22orientated generally downwardly and inwardly toward the center of thebit. Drilling fluid is pumped from the surface through fluid passage 24where it is circulated through an internal passageway (not shown) tonozzles 18 (FIG. 1). Each cutter 14-16 is typically secured on pin 20 byball bearings 26. In the embodiment shown, radial and axial thrust areabsorbed by roller bearings 28, 30, thrust washer 31 and thrust plug 32;however, the invention is not limited to use in a roller bearing bit,but may equally be applied in a friction bearing bit. In such instances,the cones 14, 15, 16 would be mounted on pins 20 without roller bearings28, 30. In both roller bearing and friction bearing bits, lubricant maybe supplied from reservoir 17 to the bearings by apparatus that isomitted from the figures for clarity. The lubricant is sealed anddrilling fluid excluded by means of an annular seal 34. The boreholecreated by bit 10 includes sidewall 5, corner portion 6 and bottom 7,best shown in FIG. 2. Referring still to FIGS. 1 and 2, each cutter14-16 includes a backface 40 and nose portion 42 spaced apart frombackface 40. Cutters 14-16 further include a frustoconical surface 44that is adapted to retain cutter elements that scrape or ream thesidewalls of the borehole as cutters 14-16 rotate about the boreholebottom. Frustoconical surface 44 will be referred to herein as the"heel" surface of cutters 14-16, it being understood, however, that thesame surface may be sometimes referred to by others in the art as the"gage" surface of a rolling cone cutter.

Extending between heel surface 44 and nose 42 is a generally conicalsurface 46 adapted for supporting cutter elements that gouge or crushthe borehole bottom 7 as the cone cutters rotate about the borehole.Conical surface 46 typically includes a plurality of generallyfrilstoconical segments 48 generally referred to as "lands" which areemployed to support and secure the cutter elements as described in moredetail below. Grooves 49 are formed in cone surface 46 between adjacentlands 48. Frustoconical heel surface 44 and conical surface 46 convergein a circumferential edge or shoulder 50. Although referred to herein asan "edge" or "shoulder," it should be understood that shoulder 50 may becontoured, such as by a radius, to various degrees such that shoulder 50will define a contoured zone of convergence between frustoconical heelsurface 44 and the conical surface 46.

In the embodiment of the invention shown in FIGS. 1 and 2, each cutter14-16 includes a plurality of wear resistant inserts 60, 70, 80 thatinclude generally cylindrical base portions that are secured byinterference fit into mating sockets drilled into the lands of the conecutter, and cutting portions connected to the base portions havingcutting surfaces that extend from cone surfaces 44, 46 for cuttingformation material. The present invention will be understood withreference to one such cutter 14, cones 15, 16 being similarly, althoughnot necessarily identically, configured.

Cone cutter 14 includes a plurality of heel row inserts 60 that aresecured in a circumferential row 60a in the frilstoconical heel surface44. Cutter 14 further includes a circumferential row 70a of nestled gageinserts 70 secured to cutter 14 in locations along or near thecircumferential shoulder 50 to cut the borehole wall. Cutter 14 furtherincludes a plurality of primary bottom-hole cutting inserts 80, 81, 82,83 secured to cone surface 46 and arranged in spaced-apart inner rows80a, 81a, 82a, 83a, respectively. Relieved areas or lands 78 (best shownin FIG. 1) are formed about nestled gage cutter elements 70 to assist inmounting inserts 70. As understood by those skilled in this art, heelinserts 60 generally function to scrape or ream the borehole sidewall 5to maintain the borehole at full gage and prevent erosion and abrasionof heel surface 44. Cutter elements 81, 82 and 83 of inner rows 81a,82a, 83a are employed primarily to gouge and remove formation materialfrom the borehole bottom 7. Inner rows 80a, 81a, 82a, 83a are arrangedand spaced on cutter 14 so as not to interfere with the inner rows oneach of the other cone cutters 15, 16.

It is common for some of the cutter elements to be arranged on conicalsurface 46 so as to "intermesh" with each other. More specifically,performance expectations require that the cone bodies be as large aspossible within the borehole diameter so as to allow use of the maximumpossible bearing size and to provide adequate recess depth for cutterelements. To achieve maximum cone cutter diameter and still haveacceptable insert protrusion, some of the rows of cutter elements arearranged to pass between the rows of cutter elements on adjacent conesas the bit rotates. In some cases, certain rows of cutter elementsextend so far that clearance areas corresponding to these rows areprovided on adjacent cones so as to allow the primary cutter elements onadjacent cutters to intermesh farther. The term "intermesh" as usedherein is defined to mean overlap of any part of at least one primarycutter element on one cone cutter with the envelope defined by themaximum extension of the cutter elements on an adjacent cutter.

Referring now to the particular construction of cutter elements, a priorart chisel insert 90 is shown in FIGS. 3A-D and a prior art conicalinsert 92 is shown in FIGS. 4A-C. As shown in these figures, the entirecutting portion of the insert is contained within the envelope of thecylindrical base portion. This is because the conventional way ofmanufacturing these inserts is by a punch and die method, which requirespositive draft at the cutting portion so as to allow the die halves toseparate after pressing operations. This restriction in manufacturingprocess imposes limitations on the geometry of the cutting portion ofthe insert. These limitations in turn prevent the optimization of thisgeometry for maximizing the bottom hole coverage and scraping actionneeded to increase rate of penetration in soft formations. Typicalpositive draft angles utilized in the manufacturing of these inserts arenot less than 10 degrees as measured per side, as shown in FIGS. 3B and4B.

The drawings show bases that are generally cylindrical, with some beingof circular cross-section and some being non-circular (e.g. oval orelliptical). However, the bases may be of any convenient cross-sectionalshape and need not be cylindrical. While the following discussion andcorresponding Figures relate to cutter inserts having cylindrical bases,it will be understood that the principles of the present invention canbe applied with equal advantage to cutter inserts having non-cylindricalbases. In cutter elements having non-circular or cylindrical bases,"positive draft" refers to instances where the entire cutting portion ofthe insert is contained within the envelope defined by projecting theshape of the base portion along the longitudinal axis of the cutterelement. As used herein, the term "longitudinal axis" refers to thelongitudinal axis of the base portion.

Referring now to FIGS. 5A-C, the chisel insert 100 of the presentinvention having an expanded geometry provides for increased mechanicalscraping/shearing action by providing increased crest length beyond thatformed on prior art inserts manufactured using conventionalmanufacturing techniques. Insert 100 includes base 102 and cuttingportion 104. The insert axis is shown as "a." Further optimization ofmechanical scraping/shearing action can be achieved with additionalexpansion of cutting portion geometry as shown in FIGS. 6A-D. As shownin FIGS. 6A-D, insert 110 has a non-circular base 112 and cuttingportion 114 which includes expanded crest 116. Using the terminologyemployed with conventional manufacturing means, this novel insert has anegative draft 114, on the cutting portion which extends beyond theenvelope "e" of the base portion. It is preferably made by themanufacturing techniques described below.

Conventional roller cone drill bits generate an uncut area on the borehole bottom known in the art as uncut bottom as shown in FIG. 3E. InFIG. 3E, the cutter elements from all rolling cone cutters are depictedin rotated profile, that is, with the cutting profiles of the cutterelements shown as they would appear if rotated into a single plane. Theuncut bottom is the area on the bore hole bottom that is not contactedby the crests of the primary row cutter elements. If this uncut area isallowed to build up, it forms a ridge. In some drilling applicationsthis ridge is never realized, because the formation material is easilyfractured and the ridge tends to break off. In very soft rock formationsthat are not easily fractured, however, the formation yields plasticallyand the ridge builds up. This ridge build-up is detrimental to thecutter elements and slows the drill bit's rate of penetration. Ridges ofrock left untouched by conventional cutting structure arrangements arereduced or eliminated by the use of the present invention as illustratedin FIG. 6E. FIG. 6E shows the reduction in uncut bottom or increasedbottom hole coverage provided by the expanded crest geometry of thecutter elements of the present invention.

To obtain the same degree of bottom hole coverage shown in FIG. 6E usingconventional cutter elements, the diameter of the base portion of thecutter elements would typically be increased to achieve thecorresponding increase in crest width. This increase in insert diameterwould have the result of reduced clearance between inserts in the samerow. as well as decreased insert-to-insert clearances between adjacentcones. To achieve adequate clearances in these areas would requiresevere compromise in insert count and placement. These compromises areavoided through the use of the present invention.

This invention is particularly suited for cutter elements used in theprimary rows where, in soft formations, maximum shearing and scrapingaction of the rock is the preferred method of cutting. Cutter elementswith elongated crests are used in these formations to provide shearingcapability. The crest width of these cutter elements inserts influencesthe aggressiveness of the cutting action relative to the formation.Thus, the function of expanded crest widths on an insert made inaccordance with the principles of the present invention can increase thevolume of shearing/scraping performed by the cutter element relative toa conventional prior art chisel insert.

Hard formations can also be addressed by this invention. Increasedcutter volume can be attained by expanding the insert extension beyondthe base while maintaining effective clearances between cutter elementsin adjacent positions in the same row and between elements in adjacentrows (both on the same cone and in different cones). With an expandedinsert extension and a reduced base diameter, insert quantities can beincreased, thereby providing greater cutter density with additionalstrikes to the formation. The increase in cutter density also providesadditional wear time for the insert, thereby extending bit life.

Depending on the shape and/or orientation of the cutter element, bottomhole coverage can be maximized to reduce or eliminate the amount ofuncut hole bottom. If the cutter elements are positioned to maximizebottom hole coverage, the number of bit revolutions necessary to gougeand scrape the entire hole bottom can be reduced 40-60% from a typicalconventional 3-cone tungsten carbide insert (TCI) rock bit.

CUTTER ELEMENT SHAPES

There are numerous variations within this invention for theconfiguration of the cutting portion of the insert that extend beyondthe envelope of the base portion. The geometry of the cutting elementcan be sculptured or non-sculptured. As used herein, the terms"contoured," "sculpted" and "sculptured" refer to cutting surfaces thatcan be described as continuously curved surfaces wherein relativelysmall radii (typically less than 0.080 inches) are not used to breaksharp edges or round-off transitions between adjacent distinct surfacesas is typical with many conventionally-designed cutter elements. Thecutting portion of the cutting element can extend up to and beyond theenvelope of its base anywhere along the perimeter of the base portionand any multitude of times. The preferred manufacturing techniquesdescribed below allow for new insert shapes that extend up to and beyondthe "envelope" of the base portion of the insert thereby opening thedoor for countless new geometries. Several embodiments of the inventionas applied to insert type cutter elements are illustrated in FIGS. 5through 18. For example, in some preferred embodiments, the longitudinalaxis of the of cutter element does not intersect the crest of the cutterelement. Like the embodiments shown in FIGS. 5A-C, 6A-D, theseembodiments incorporate the principles of the present invention. Foreach embodiment in FIGS. 7 through 18, the comments in Table I set outthe mechanical advantages that are believed to result from the specificfeatures of that embodiment.

                  TABLE I                                                         ______________________________________                                        Figure                                                                        Number  Insert Description                                                                          Comment                                                 ______________________________________                                        FIG. 7A-C                                                                             Offset crest chisel                                                                         Optimize aggressive scraping                                    with negative draft.                                                                        action in specific applications.                        FIG. 8A-C                                                                             Offset crest chisel with                                                                    The reinforcement rib provides                                  negative draft and                                                                          increased support to improve                                    reinforcement rib.                                                                          durability when drilling through                                              hard stringers.                                         FIG. 9A-C                                                                             Offset conical with                                                                         Optimize scraping action in                                     negative draft.                                                                             non-plastic formations.                                 FIG. 10A-C                                                                            Biased negative draft                                                                       Optimize scraping action where                                  chisel.       insert - to - insert clearances                                               between cones is constrained.                           FIG. 11A-C                                                                            Partial biased negative                                                                     Optimize scraping action where                                  draft chisel. insert to insert clearances between                                           cones is constrained.                                   FIG. 12A-C                                                                            Arc crest chisel with                                                                       Structural support for insert crest/                            zero draft.   corners and improved scraping                                                 action.                                                 FIG. 13A-C                                                                            Arc crest chisel with                                                                       Structural support for insert crest/                            negative draft.                                                                             corners and optimized scraping                                                action.                                                 FIG. 14A-C                                                                            Spline crest chisel with                                                                    Structural support for insert crest/                            zero draft.   corners and improved scraping                                                 action.                                                 FIG. 15A-C                                                                            Spline crest chisel with                                                                    Structural support for insert crest/                            negative draft.                                                                             corners and optimized scraping                                                action.                                                 FIG. 16A-C                                                                            Partial negative draft                                                                      Insert chisel crest corner protection                           chisel.       for tougher applications.                               FIG. 17A-C                                                                            Offset crest chisel with                                                                    Aggressive positive rake for                                    negative draft on                                                                           maximum formation removal.                                      leading flank.                                                        FIG. 18A-C                                                                            Slant crest chisel with                                                                     Increased unit load upon entering                               negative draft.                                                                             the formation to maximize                                                     penetration.                                            ______________________________________                                    

CUTTER ELEMENT PLACEMENT

Further optimization of the cutter elements of the present invention canbe achieved by their orientation and placement within the cone bodies.This will further maximize the desired level of scraping action forincreased mechanical efficiency.

Referring to FIG. 21, novel inserts 110 are shown placed in aconventional orientation in a row 110a with the axis of each insertbeing coplanar with the cone axis. Another arrangement is shown in FIG.22, in which each insert 110 is oriented in the cone body such that theaxis "a" of the cylindrical portion of the insert is offset a distance"D" with respect of the cone axis. This further gives the designerflexibility to optimize the scraping action with regards to the specificformation and application.

FIGS. 23 and 23A show another orientation, wherein the crest 116 of theinsert 110 is rotated about the insert axis "a" such that an angle α isdefined with respect to the projection of the cone axis, as best seen inFIG. 23A. It will be understood that in certain applications, it may beadvantageous to rotate one or more inserts in the opposite directionsuch as by an amount α'. FIG. 24 shows still another embodiment, whereinthe insert 110 is both offset a distance "D" and rotated about its axis"a." Any of the inserts shown in FIGS. 5-18 and FIGS. 25-31 (discussedbelow) can be employed in the arrangements or orientations shown inFIGS. 21-24. The cutter elements 110 can be mechanically ormetallurgically secured into the cone by various methods, such as,interference fit, brazing, welding, molding, casting, or chemicalbonding. The inserts described in the FIGS. 5 and 7-18 and orientations21-24 are shown with a cylindrical 4e base portion for interference fitinto a matching socket. The base portion of each insert need not becircular, however, as shown in FIGS. 6A-D.

INSERT MATERIAL TYPES

An insert of the present invention can be made of tungsten carbide andin addition can be partially or fully coated with a "superabrasive"(i.e., a material having a hardness of at least 2,700 Knoop kg/mm²) suchas PCD, PCBN, etc.

BIT DESIGN INTENT

Depending on the bit design objectives, the amount of uncut bottom canbe reduced or eliminated. Currently, most bits are designed with cutterintermesh between the rolling cones, which can invoke limitations on thewider crest of the cutter elements. Hence, designing bits withoutintermesh can allow greater latitude in crest width. The cutter elementsof the present invention can be used in bits that have intermeshedcutter elements, as well as in those that do not.

Additionally, these cutter elements can be used in all types of rollingcone bits having one, two or more rolling cones.

The increased bottom hole coverage attainable with the present inventionpermits the use of fewer rows of cutter elements on the cone cutters ofthe bit. Having fewer rows of cutter elements, as compared toconventional prior art bits, increases the unit loading per cutterelement thus increasing rate of penetration. For example, in oneconventional 3-cone TCI roller cone bit, a total of nine rows of primarycutter elements dispersed among the three cones were employed to cut thebottom hole as shown in rotated profile in FIG. 3E, there being threerows, specifically Rows 7,8 and 9, aligned in the same rotated profileposition. Using the expanded crest geometry of the present invention,and as shown in rotated profile FIG. 37, the bottom hole coverage can beattained using only a total of 8 rows of cutter elements on this 3-conebit. Thus, the present invention allows TCI bits to be designed with 8or fewer rows, in contrast to conventional prior art TCI bits, whichtypically have 9 or more rows.

INSERT MANUFACTURING TECHNIQUES

Conventional rolling cone bit inserts are manufactured by press and dieoperations. As shown in FIG. 19, the top and bottom dies 8, 3 arepressed axially, to form an insert 1 with a cylindrical base 9 and acutting element extension 2, contained within the envelope of thecylindrical base. Positive draft must be designed into the extensionwithin the constraints of the cylindrical base. Draft refers to thetaper given to internal sides of a closed-die to facilitate its removalfrom the die cavity. To complete the conventional insert 1, a centerlessgrind operation is performed on the base portion 9 to provide specifiedcylindrical geometry and surface finish. In centerless grinding theinsert 1 is supported on a work rest and fed between the grinding wheeland a rubber bonded abrasive regulating wheel. Guides on either side ofthe wheels direct the work to and from the wheels in a straight line.

When inserts have extension geometries that extend out to and beyond theenvelope of the cylindrical base as contemplated by the presentinvention, conventional manufacturing techniques such as axial insertpressing and centerless grinding cannot be used. Techniques have beenand are being developed to provide the ability to create the novelinserts of the present invention such as those shown in FIGS. 5-18. Forexample, instead of pressing each insert along the longitudinal axis ofits base "a," the inserts of the present invention (such as insert 110)can be pressed normal to that axis, as shown in FIG. 20, thus creatingsides instead of a top and bottom. The present insert 110 can also bemanufactured by injection molding, multi-axis CNC milling machine, wireEDM, casting, stereolithography or other free-forming methods.

The insert base portion 112 can be finished by using other grindingmethods post grinder, in-feed centerless grinder) or by single pointmachining (turning).

NON-RECTILINEAR CRESTS

A further aspect of the present invention involves the use ofnon-rectilinear crests on cutter elements. FIGS. 12, 13, 14 and 15comprise examples of non-positive draft cutter elements havingnon-rectilinear crests. Specifically, the cutter elements shown in FIGS.12, 13, 14 and 15, have non-rectilinear crests when viewed along theirlongitudinal axes. Non-rectilinear crests are defined as crests that areelongated and curvilinear in nature when viewed along the longitudinalaxis "a" of the inserts, as shown in FIGS. 12A, 13A, 14A, and 15A. It ispreferred that the non-rectilinear crest also be substantially uniformin width when viewed along longitudinal axis "a," however crests havingnon-uniform widths are contemplated as being within the scope of thepresent invention.

It has been discovered that these non-rectilinear crests, sometimesreferred to herein as "curvilinear crests," have distinct advantages insome formations. The advantages of the curvilinear crests are realizedin cutter elements having positive drafts, as well as in thenon-positive draft cutter elements described above. Specifically,referring to FIGS. 25A-D through 31A-D, various preferred embodiments ofcutter elements having curvilinear crests and positive drafts are shown.

Referring initially to FIG. 25A-D, a cutter element 300 having anarcuate crest 302 and a positive draft is shown. The lines of thearcuate crest 302 are continued down the leading and trailing faces ofcutter element 300, resulting in a concave leading face 304 and a convextrailing face 306. As shown, the left and right sides of cutter element300 are symmetrical, but it will be understood that they could beasymmetrical and still achieve the desired features. Likewise, and asdiscussed above, the arcuate crest can be used in conjunction withcutter elements having zero or negative drafts as well.

Referring now to FIG. 26, a cutter element 310 having an S-shaped crest312 and a positive draft has a leading face 314 that is both partiallyconcave and partially convex and a trailing face 306 that is alsopartially concave and partially convex. Again, the sides of the cutterelement are shown as being symmetrical, but could also be asymmetricalif desired. Also, the S-shaped crest can be used in conjunction withcutter elements having zero or negative drafts.

In FIG. 27, an alternative embodiment of the cutter element of FIG. 26is shown as 320, wherein the curves of the S-shaped crest are reversedas compared to FIG. 26. This results in a reversal of the convex andconcave portions of the leading and trailing faces 324, 326respectively. While this embodiment is shown having a positive draft, itcould be applied with equal advantage to cutter elements having zero ornegative drafts as well.

FIG. 28 shows a cutter element 330 having a J-shaped crest 332, whichresults in the concave portion of the leading face and the convexportion of the trailing face being off-center and closer to one side ofthe cutter element as drawn. Similarly, in FIG. 29, a cutter element 340has a J-shaped crest, in which the convex and concave portions areoffset in the opposite direction as compared to cutter element 330.

FIGS. 30A-D show a cutter element 350 having an arcuate crest 352 thatis inclined with respect to the longitudinal axis of the insert. Thisinclination is not typically perceptible in the top view (view 30A), butcan be seen in the front view (view 30B). While the other curvilinearcrest shapes described above are shown with substantially straight orbowed shapes, it will be understood that any the curvilinear crests canbe canted with respect to the axis of the cutter element base.Similarly, FIGS. 31A-D show an insert 360 having an arcuate crest 362, aconcave leading face 364, and a convex trailing face 366 when viewedalong the longitudinal axis of the insert, as shown in FIG. 31A. Theconcave/convex combination fictions, to increase the bending strength ofthe extending portion of the cutter element.

The various features described above, including curvilinear crests,inclined or bowed crests, concave and convex faces and variations indraft can be combined and optimized to provide improved wear resistanceand enhanced ROP, depending on the formation and other structure. Eachof these concepts can also be applied with equal advantage to cutterelements having positive, zero and negative drafts. Both the durabilityand ROP potential of a curvilinear crest insert can be substantiallyimproved as compared to a similarly proportioned conventional insertdesigned with a straight or linear crest.

For example, as the roller cone rotates, the crest of each insert firstengages the formation when the longitudinal axis of the insert is notperpendicular to the formation surface. This non-perpendicular loadingcondition induces bending or tensile stresses in the insert, which cancause chipping and/or breakage of the cutting element. These stressesare particularly pronounced at the "comers" of the crest (i.e. theopposing ends of the crest). By introducing a curvilinear crest (e.g.C-, S-, or J-shape), one or both of the comers of the insert are offsetfrom the longitudinal axis toward the leading side. These offset comersof the insert initially engage the formation when the axis of the cutterelement is closer to perpendicular. Hence, the reactive forces from theformation reduce tensile stress in the insert at initial engagement whenimpact forces/loads on the insert are at their maximum. This isadvantageous, as tungsten carbide has a very high compressive strengthand relatively lower tensile strength. Additionally, this improved"angle of initial penetration" provides a more aggressive and efficientcutting action that can improve ROP. Offsetting the crest comers alsocreates a convex trailing side surface, in planes both parallel andperpendicular to the longitudinal axis, which adds more carbide mass forsupport of reactive forces from the formation.

As the insert crest continues into the formation, the "C" shaped crestimproves the stress distribution within the insert. Any forces appliedto the crest comer will have a force component vector directed along atangent to the curvilinear centerline of the crest. This force componentreduces the force component perpendicular to the crest, which is againstthe weaker moment of the cross-sectional area in the local region,thereby reducing the tensile stress of bending.

In addition, the concavity in the leading side (in a plane perpendicularto the longitudinal axis) mechanically lifts the formation from the holebottom, instead of forcing or plowing the formation to the side(s). Thisallows drilling fluid to penetrate beneath the resultant "chip" andreduces the hold-down force applied by the drilling fluid column. Forthese reasons, the present curvilinear shapes are particularlyadvantageous in plastic formation types.

By way of another example, the spline, or S-crested cutter elements ofFIGS. 14, 15, 26 and 27 have cutting leading and trailing faces thateach include both concave and convex portions. This creates a twistingforce as each cutter element penetrates and then translates through intothe rock. This causes the rock to fail in tension rather thancompression, increasing ROP.

While various preferred embodiments of the invention have been shown anddescribed, modifications thereof can be made by one skilled in the artwithout departing from the spirit and teachings of the invention. Theembodiments described herein are exemplary only, and are not limiting.Many variations and modifications of the invention and apparatusdisclosed herein are possible and are within the scope of the invention.Accordingly, the scope of protection is not limited by the descriptionset out above, but is only limited by the claims that follow, that scopeincluding all equivalents of the subject matter of the claims.

What is claimed is:
 1. A drill bit for cutting a formation, comprising:abit body having a bit axis; a plurality of rolling cone cuttersrotatably mounted on cantilevered bearing shafts on said bit body, eachrolling cone cutter having a generally conical surface; a firstplurality of primary cutter elements extending from a first of said conecutters in a first row, said first row extending to less than full gage;a second plurality of primary cutter elements extending from a secondcone cutter in a second row, said second row extending to less than fullgage, said second primary cutter elements intermeshing with said firstprimary cutter elements; and at least one of said primary cutterelements having a non-rectilinear crest and a base portion adapted tofit into a corresponding socket on a rolling cone cutter.
 2. The bit inaccordance with claim 1 wherein said crest is arcuate.
 3. The bit inaccordance with claim 1 wherein said crest is a substantially S-shapedspline.
 4. The bit in accordance with claim 16 wherein said cutterelement has a leading face that includes both concave and convexportions when viewed along the longitudinal axis of the cutter element.5. The bit in accordance with claim 1 wherein said crest is J-shaped. 6.The bit in accordance with claim 1 wherein said cutter element has aconcave leading face and a convex trailing face when viewed along thelongitudinal axis of the cutter element.
 7. The bit in accordance withclaim 1 wherein said cutter element has a longitudinal axis and saidlongitudinal axis is offset such that it does not intersect the axis ofsaid cone.
 8. The bit in accordance with claim 1 wherein said base isnon-cylindrical.
 9. The bit in accordance with claim 1 wherein said baseis non-circular.
 10. The bit in accordance with claim 1 wherein at leasttwo cutter elements have non-rectilinear crests.
 11. The bit inaccordance with claim 1 wherein at least one cutter element has anextending portion having zero draft.
 12. The bit in accordance withclaim 1 wherein at least one cutter element has an extending portionhaving negative draft.
 13. The bit in accordance with claim 1 wherein atleast one cutter element has an extending portion having positive draft.14. The bit in accordance with claim 1 wherein at least one cutterelement has an extending portion having a con toured surface.
 15. Thedrill bit according to claim 1 wherein said cutter element has alongitudinal axis and said longitudinal axis does not intersect saidcrest.
 16. A drill bit comprising:a bit body; at least two roller conesrotatably mounted on a cantilevered bearing shaft depending from saidbit body; a first plurality of primary cutter elements extending from afirst of said roller cones in a first row, said first row extending toless than full gage; a second plurality of primary cutter elementsextending from a second roller cone in a second row, said second rowextending to less than full gage, said second primary cutter elementsintermeshing with said first primary cutter elements; and at least onecutter element extending in a primary row from a roller cone, saidcutter element having a base portion and an extending portion, saidextending portion having a non-rectilinear crest and extending beyondthe envelope defined by said base portion.
 17. The bit in accordancewith claim 16 wherein said crest is arcuate.
 18. The bit in accordancewith claim 16 wherein said crest is a substantially S-shaped spline. 19.The bit in accordance with claim 16 wherein said cutter element has aleading face that includes both concave and convex portions when viewedalong the longitudinal axis of the cutter element.
 20. The bit inaccordance with claim 16 wherein said crest is J-shaped.
 21. The bit inaccordance with claim 16 wherein said cutter element has a concaveleading face and a convex trailing face when viewed along thelongitudinal axis of the cutter element.
 22. The bit in accordance withclaim 16 wherein said base is non-circular.
 23. The bit in accordancewith claim 16 wherein at least two cutter elements have non-rectilinearcrests.
 24. The bit in accordance with claim 16 wherein said extendingportion has a contoured surface.
 25. The drill bit according to claim 16wherein said cutter element has a longitudinal axis and saidlongitudinal axis is offset such that it does not intersect the axis ofsaid cone.
 26. The drill bit according to claim 16 wherein said cutterelement has a longitudinal axis and said longitudinal axis does notintersect said crest.